We have continued to use highly enriched preparations of sarcolemma and lysosomes as in vitro models for the study of membrane injury. The hydrolysis of structural lipids of sarcolemma-enriched membranes by endogenous lipases results in the release of free fatty acids from neutral lipids and phospholipids; lysophosphatidyl choline is the predominant phospholipid metabolite. When these membranes are attacked by an exogenous phospholipase A2 from granulocytes lysophosphatidyl ehtanolamine is the major phospholipid metabolite and arachidonic acid is the predominant fatty acid. Incubation of sarcolemmal membranes, purified 100-fold from adult cardiac myocytes, with soluble lipases from purified lysosomes results in significant release of free fatty acids and, in the presence of detergent, dramatic hydrolysis of sphingomyelin. Highly pure lysosomes contain lipases that produce lysophospholipids during autolytic digestion; inhibition of these lipases with chlorpromazine protects against the loss of latency during incubation at acid pH. The above studies have provided additional data about the relevance of lipolytic enzymes in the "in vitro injury" of lysosomes and sarcolemma.